1. Field of the Invention
The present invention relates to a communication system configured to perform an orthogonal frequency division multiplexing (OFDM) transmission.
2. Description of the Related Art
An OFDM transmission system is a system for transmitting an information signal in parallel using a subcarrier orthogonally arranged on a frequency axis. In the OFDM transmission, propagation path conditions, e.g., a ratio of reception signal power to noise power and a ratio of desired signal power to undesired signal power, are generally different for every subcarrier and are also temporarily varied. Therefore, transmission efficiency can be improved by properly selecting a primary modulation system and modulation parameters, e.g., an error correction code and a guard interval (GI) length, according to the propagation path conditions. A transmission mode can be flexibly changed by selecting the modulation parameters according to a request of transmission data. For example, data can be reproduced with high accuracy by changing a symbol length according to an output cycle of video data or sound data and handling symbol timing as reproduction timing in a receiving apparatus.
When a transmission apparatus transmits a modulation parameter that changes the modulation parameter in such a way, the transmission apparatus needs to notify the modulation parameter at a time of changing to a receiving apparatus so that the receiving apparatus can correctly perform demodulation processing. Particularly, when a parameter relating to an operation of fast Fourier transform (FFT), e.g., changing of a symbol length, is changed, the receiving apparatus needs to extract a modulation parameter before receiving an effective symbol of a next symbol.
Japanese Patent Application Laid-Open No. 08-088617 discusses a technology which multiplexes a modulation parameter with a control symbol provided at a predetermined position in a transmission frame and transmits it. In this technology, a transmission apparatus transmits the modulation parameter by changing an envelope of the control symbol. A receiving apparatus extracts the modulation parameter from the control symbol and then performs a demodulation processing corresponding to the parameter. In this case, since the modulation parameter is extracted by low processing delay without performing a frequency analysis by FFT, it is possible to change an operation relating to FFT of the next symbol.
However, since the method discussed in Japanese Patent Application Laid-Open No. 08-088617 needs to provide a control symbol for transmitting a modulation parameter in a transmission frame, there is a problem that the method significantly squeezes a transmission band when a frequency of change of the modulation parameter is high.